Ordinary Portland Cement is one of the basic binder ingredients of concrete and mortar and is a controlled chemical combination of compounds made from calcium, silicon, aluminum, and iron oxides, along with small amounts of other materials. These compounds are typically formed using naturally occurring materials. Ordinary Portland Cement may have four primary phases: tricalcium silicate (“C3S”) (Ca3SiO5); dicalcium silicate (“C2S”) (Ca2SiO4); tricalcium aluminate (“C3A”) (Ca3Al2O6); and tetracalcium aluminoferrite (“C4AF”) (Ca4Al2Fe2O10). Other cementitious materials may also be used as one of the basic binder ingredients of concrete and mortar.
The raw materials used to produce cement may be limestone, clay, shale, sand, or iron ore. The current manufacturing process may consist of proportioning the raw materials to the correct chemical composition and grinding them to a fine consistency. The ground material is then fed to a rotary kiln, which is a large cylindrical continuously rotating furnace, and heated in the 1500 to 1600° C. range. The raw materials are calcined, become partially molten, and react to form the four complex compounds shown above. These compounds exit the kiln as a hard, sintered agglomerate form called “clinker.” The clinker is cooled, mixed with approximately 5% gypsum, and ground into its final powder form. Gypsum (calcium sulfate, CaSO4.2H2O) is a necessary additive that helps regulate the setting time of concrete and in this respect becomes the fifth major ingredient of cement. Without the inclusion of gypsum, the hydration rate of the calcium aluminate phase, C3A, is too fast and would not allow enough time to “work” or “place” a concrete mixture before setting. An exemplary preparation process in shown in FIG. 1.
The chemical composition of cement is what distinguishes one type from another. Typical ASTM Type I Portland cement comprises approximately the following percents by weight: 50% C3S, 25% C2S, 10% C3A, 10% C4AF, and 5% gypsum. However, the industry usually identifies the cement by the amount of oxides in the raw materials, such as lime (CaO) and silica (SiO2). Lime and silica make up about 85% by mass of the final product. When the four primary cement phases are listed as basic oxides Ordinary Portland Cement approximately comprises the following percents by weight: 64% CaO, 22% SiO2, 6% Al2O3, and 3% Fe2O3.
Concrete is produced by mixing cement with fine aggregate (sand), coarse aggregate (crushed stone), water, and possibly small amounts of additives to alter the properties. For example, a concrete mix may contain the following by weight: about 12% Portland cement, about 34% sand, about 48% crushed stone, and about 6% water. The setting and rate of strength development of concrete can be varied by the use of alternate cement compositions as designated by ASTM. Compositions for the major 5 types of ASTM Portland cement are shown in Table 1. In addition to the composition shown for each type of Portland cement, the mixture also contains an additional 5-7% ground gypsum.
Mortar, on the other hand, is the binder material used to both fill the gaps and to bond between “blocks” used in construction. These blocks may be stone, brick, cinder block, manufactured concrete shapes, etc. The primary ingredient in mortar is the same Portland cement used in concrete. Mortar is essentially a mixture of sand, Portland cement (sometimes with and/or without lime), and water. It is applied as a paste which then sets hard in a similar fashion as concrete. Mortar is literally the glue that holds the wall system together. It can also be used to fix masonry when the original mortar has been washed away. Even though mortar makes up as little as 7% of the total volume of a masonry wall, it plays a crucial role in the performance of the structure. It not only bonds the individual units together, but it also seals the building against moisture and air penetration. While compression strength and durability are critical properties of concrete, bond strength and durability are the critical properties of mortar.
When water is added to cement, each of the compounds undergoes a hydration reaction that forms the cementatious gel that surrounds and holds or binds the additional aggregate materials together and to form a strong solid. Each of the primary phases contributes to the overall physical and chemical characteristics of the final cementatious product.
TABLE 1Ordinary Portland Cement Compositions and Rate of Strength DevelopmentCompressiveStrength as % ofASTM TypeCompositionType I CementASTM DesignationC3SC2SC3AC4AF1 Day2 Days28 DaysIGeneral Purpose5024118100100100IIModerate Heat4233513758590and SulfateResistanceIIIHigh Early601398190120110StrengthIVLow Heat2650512555575VSulfate Resisting404049657585
A significant amount of energy is required to manufacture traditional cement. For example, in the US, plants required an average of about 5.0 MMBtu/tonne of cement manufactured with the most energy efficient plant requiring about 3.2 MMBtu/tonne. The average rotary kiln operation uses about 93% of the total energy, with clinker grinding requiring approximately 5%. In addition, the rotary kilns may operate at only 34% energy efficiency. Another downside to the current cement manufacturing process is the exhaustion of CO2 emissions. For every tonne of cement produced in a rotary kiln, approximately 1 tonne of CO2 is released to the atmosphere from the CO2 content of the starting limestone and in the combustion of fossil fuel that provides the heat needed for clinker production.
Thus, there remains a need in the art for more energy efficient methods of producing cement. Additionally, there remains a need in the art for cement compositions that can provide concrete exhibiting improved properties.